This invention relates to solid-state smoke detectors and smoke (fire) alarm systems, and more particularly to a novel and improved smoke-sensitive element comprising a bismuth oxide film deposited such as by electron beam evaporation, sputtering, or otherwise, by means well known in the art, on one surface of an electrically insulating material and a pair of electrodes. It also relates to a method of depositing this oxide on a substrate and of heat treatment which provides for a highly smoke-sensitive film which does not require additional heating elements during operation. This invention also relates to smoke detector signal processing circuits.
A small, inexpensive smoke detector capable of operating with high sensitivity, quick response, extremely low power consumption, high accuracy, and reliability is desirable. Prior art detectors have used heating to ensure the desired sensitivity of the detector element. However, heating the element consumes a significant amount of power and can lead to instability in some of the sensitivity characteristics of the smoke detecting element.
Many smoke detectors operate upon the principle that an ionization current catalyzed by a radiation source is affected by the presence of particulates such as those found in aerosols or smoke emanating from a fire. The particulates change the ionization current and the change in current can be detected and correlated with the density of the particulate matter to provide a fairly accurate indication of a smoke condition.
A radioactive source placed in an ionization chamber can be used as the radiation source, as disclosed for example in U.S. Pat. No. 4,037,206 and as discussed in the book "Fire Safety, Explosion Safety" edited by Baratov (1987). Such ionization smoke detectors are currently produced by industry and can be powered with a battery. However, it is necessary to be very careful with such smoke detectors in living accommodations, hotels and so on.
Optical smoke detectors utilize the change in air transparency due to the existence of smoke to detect smoke. The light for an optical smoke detector may be provided for example by a tungsten lamp or a light-emitting diode (LED) and the light sensor may be a photosensitive semiconductor device. For examples of optical smoke detectors see Baratov (1987). However, optical smoke detectors are complicated and may contain many different electronic components. Additionally, precautions must be taken to minimize the chance of a false alarm, since a false alarm may be triggered by a false light signal from the sun, a lamp or some other extraneous source of light or even a change of air transparency caused by dust.
Different types of metal-oxide semiconducting gas sensors are well known. There are numerous kinds of gas sensing elements, for example, thin-film, thick-film, sintered and ceramic. In order to enable the gas sensing elements to effectively detect a gas it is usually necessary to heat the element in operation in order to increase its sensitivity to a useful level. There are several methods for heating the sensitive element and for fabricating the sensitive element, reference sensor unit and heater. In some cases a catalytic layer or catalytic islands are deposited on the substrate to enhance sensitivity. The details of such gas sensing elements are discussed in many publications, for example, Aroutiounian (1991) and U.S. Pat. Nos. 4,338,281, 4,343,768, 4,574,264, 4,580,439, 4,911,892 and 4,928,513.
The following two patents, U.S. Pat. Nos. 3,900,815 and 4,016,524 disclose, for example, semiconductive elements for smoke detectors. In accordance with these patents, the semiconductor is covered with a dielectric film formed with a large number of microscopic openings or pores. In Nitta & Terada (1982) the authors disclose a porous p-type semiconductive MgCr.sub.2 O.sub.4 TiO.sub.2 ceramic as the gas-sensitive material. Its electrical conductivity changes with different gas chemisorption. Sensitivity of such a ceramic to smoke is remarkably small compared to its sensitivity to H.sub.2 S and various organic molecules containing functional radicals (CH.sub.3 CHO, C.sub.2 H.sub.5 OH, etc.) in the range of heating temperatures up to 550.degree. C. Another multifunctional (smoke and different gases) detector with heater and catalyst metal layer is disclosed in U.S. Pat. No. 4,569,826.
One problem with prior art detectors is that they can be sensitive to a large number of different gases and limiting the sensitivity can be a complex process. As is disclosed in the above mentioned publications, the same metal-oxide body with different catalyst metal layer or doping, together with a multilayer film can be used for detecting a number of different gases; in the case of SnO.sub.2, the number of gases detected can reach more than 20. To provide for selectivity of the detector it is necessary to change the heating temperature and dope the semiconductor body with different impurities or oxides. Regretfully, even today, there are problems with the selectivity of semiconductor detector and the rather high electric power consumption in the waiting mode or idle, non-alarm, state during the detector's operation.
Researchers have investigated different bismuth-containing materials. Influence of the dielectric Bi.sub.2 O.sub.3 thin layer on properties of semiconductive metal-oxide-GaAs solar cells is discussed in Wang & Pandeliser (1981) and Pandeliser & Wang (1982). There are other Bi.sub.2 O.sub.3 applications, but bismuth-containing materials are rarely used for the purpose of gas detector fabrication. The complex oxide, bismuth molybdenum oxide is disclosed in U.S. Pat. No. 4,307,373 as having an electrical conductivity dependent on the presence and/or concentration of a gas or gases in the ambient atmosphere. Bismuth ferrites BiFeO.sub.3, Bi.sub.4 Fe.sub.2 O.sub.9 and Bi.sub.2 Fe.sub.2 O.sub.9, as well as, Bi.sub.4 V.sub.2 O.sub.11 are disclosed as being useful as sensitive compounds for detecting acetone, ethanol or gasoline vapors and natural gas in USSR Patent No. 1,569,689 and in Pogossian & Abovian (1991). As disclosed in Hykaway & Sears (1988), bismuth molybdate evaporated films are sensitive to alcohols, ketones, alkenes, hydrogen, carbon monoxide and water vapors. However, the operating temperature of the prior art bismuth containing gas sensitive compounds was much higher than the room temperature.
The possibility of using ultrafine particles of a metal oxide as a gas sensitive material where the oxide of Bi or seventeen other metals may be made is disclosed in U.S. Pat. No. 4,313,338 ("the '338 patent"). The '338 patent focused on detectors based on SnO.sub.x and CuO.sub.x which detected isobutane and ethyl alcohol and also could be used as humidity sensors. However, it was necessary to heat the disclosed sensitive films to at least above 100.degree. C. (see, for example, FIGS. 4, 10, 12 and 13, the '338 patent). In addition, the oxide had to be cerated by a special method in oxygen gas plasma (see, for example, FIGS. 11, 14, 15, the '338 patent) with ultrafine particles having a mean diameter of between ten and several hundred angstroms. These particles should be small monocrystals of a mixture of appropriate metals (Sn or Cu) with its oxides (SnO, SnO.sub.2 or CuO, Cu.sub.2 O). After preparing such a gas-sensitive film of ultrafine particles (SnO.sub.2 --SnO--Sn, for example) in a boat in one part of the evaporator in an oxygen atmosphere while in the other boat in the same evaporator an extra thin film of ultrafine particles of palladium having a mean particle diameter of several ten angstroms and having a thickness of about 2 to 5 millimeters was deposited in an inert gas medium. It was also disclosed that in obtaining measurements of sensor characteristics an alternating current power source was used.
Besides listing bismuth, the '338 patent does not disclose any information about bismuth, or the other fifteen oxides, nor does it disclose any information about sensitivity to smoke or other gases. The '338 patent discloses the use of a heating means and an aggregate of ultrafine Sn or Cu particle oxides and palladium. Moreover, the '338 patent, and the other above mentioned patents all disclose heating the sensitive film body in operation and most of them disclose the necessity of coating the sensitive film layer with islands or particles of catalyst metal or their oxides.
UK Patent No. GB2043913A discloses a gas-sensitive element whose electrical resistance at room temperature is dependent upon the concentration of a reducing gas to which it is exposed. It comprises of a major proportion of indium oxide and a minor proportion (2 to 12.5%) of platinum black. It is made by grinding a mixture of indium oxide or an indium salt decomposible to it and platinum black or a platinum compound decomposible to it, baking and sintering. This patent does not disclose the sensitivity of this material to smoke, nor does it eliminate the general deficiency of the prior art devices of requiring the use of noble and expensive metals such as platinum or palladium.
Signal processing circuits diagrams for sensors are well known (see, for example, Chorwitz and Hill and UK Patent No. GB2043913). The sensor (and the smoke-sensitive element in particular) connects usually in a bridge circuit which gets out of balance when the smoke appears in the air atmosphere. Such circuits are complicated, have low sensitivity and noise immunity, and consume rather large amounts of electrical power in the waiting mode.
The prior art does not teach that a resistive film of bismuth oxide can operate without heating as the sensing material for a smoke (fire) detector. The prior art does not teach depositing bismuth oxide on an electrically insulating substrate as a resistive film and supplying a pair of electrodes so as to provide a smoke (fire) sensitive element and/or device. The prior art does not teach the regimes of deposition and heat treatment for fabrication of the necessary modification of bismuth oxide as taught in the present invention, as well as signal processing circuits which are necessary for reliable and effective operation of a smoke detector based on this particular sensitive element.